1. Field of the Invention
This invention relates to an abnormality-detecting device for detecting abnormality of an exhaust gas component concentration sensor arranged in an exhaust system of an internal combustion engine.
2. Prior Art
Conventionally, an abnormality-detecting device for detecting abnormality of an exhaust gas component concentration sensor arranged in an exhaust system of the engine has been proposed by the present assignee e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 4-233447, in which feeble current is caused to flow through the exhaust gas component concentration sensor to check an amount of change in the output therefrom, whereby it is determined from the detected amount of change in the sensor output whether there is an abnormality of the sensor, such as a disconnection and a short-circuit, or an abnormality due to aging or the like.
As shown in FIG. 16, a typical exhaust gas component concentration sensor of this kind is comprised of a sensor element 51 formed of a solid electrolyte of zirconia (ZrO.sub.2) arranged within a casing 52. More specifically, the sensor element 51 in the form of a tube has inner and outer surfaces thereof coated with platinum as an electrode. Exhaust gases are introduced into a space 53 defined between the outer surface of the sensor element 51 and the casing 52, while fresh air as a reference gas is introduced into an inner space 54 defined by the inner surface of the sensor element 51. The sensor has a characteristic that an electromotive force thereof drastically changes as the air-fuel ratio of the exhaust gases changes across a stoichiometric air-fuel ratio, so that an output signal thereof is inverted in level from a leaner side to a richer side with respect to the stoichiometric air-fuel ratio and vice versa. More specifically, a high level output signal assumes when the air-fuel ratio is rich, and a low level when the same is lean.
Further, the exhaust gas component concentration sensor is provided with a heater 55 arranged in the inner space 54 of the sensor element 51 for accelerating the activation of the sensor.
However, in the exhaust gas component concentration sensor, when the exhaust system and the exhaust gas component concentration sensor are cold, e.g. when the engine is started in a cold condition, or when the ambient air is low in temperature and/or high in humidity, there can be formed water condensed from vapor contained in exhaust gases, which stays on the sensor element 51 and the casing 52 in a fashion bridging therebetween, as indicated by X in FIG. 16. This causes a drop in the electric resistance of the sensor element 51. As a result, when subtle current is caused to flow through the sensor element 51 with condensate water thereon, an apparent short-circuit is caused by the bridge of the condensed water, so that there occurs no change in output voltage of the sensor. This leads to detection of abnormality of the sensor. That is, in spite of the fact that when the engine has been warmed up, the condensed water evaporates to cancel the apparent short-circuit of the sensor element, to bring the sensor into a normally functioning state in which the sensor delivers a normal output voltage, the sensor is erroneously detected to be abnormal when the engine is started under the aforementioned environmental condition (cold condition).